Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume printing press to produce an example of an intended image. These intended images may require several corrections and be reproduced several times to satisfy customers' requirements, which results in loss of profits. By utilizing pre-press color proofing, time and money can be saved. Ideally when using a proof to simulate the press-sheet the proof is screened at the same halftone screen rulings, screen angles, and dot shape. It is even better if the same raster image processor (RIP), is used to generate the screens and the writing resolution of the proofer and the plate writer are the same. These are the ideal conditions to guarantee that the proof is a reliable simulation of the press sheet.
A direct digital color proofer such as the one described in U.S. Pat. No. 5,164,742 images a bitmap of a halftone screen using a plurality of laser diodes at a high resolution. The Kodak Approval direct digital color halftone proofer introduced for sale in 1991 wrote at 1800 dots per inch and imaged halftone screens from 65 lines per inch to 300 lines per inch in each of cyan, magenta, yellow, and black. The original Kodak Approval Classic had eighteen writing lasers and two dummy channels. The Kodak Approval XP4, direct digital color halftone proofer, introduced in 1998 is available in either 2400 dots per inch or 2540 dots per inch writing resolution. The Kodak Approval XP series printers use up to 28 writing channels and two outside dummy channels. In 1999 Kodak introduced recipe color software that enabled the Approval family of proofers to image multiple donors using the same bitmap thereby mixing the colorant of the halftone screen in the proof. Today a screen set may include halftone bitmaps for cyan, magenta, yellow, black, orange, green, red, blue, white, metallic, and recipes. Each bitmap will have an associated screen ruling, screen angle, and RIP ID. The RIP ID is the make and model of the raster image processor that created the bitmap plane. The Kodak Approval system currently accepts RIP input from Harlequin Scriptworks, Heidleberg, Barco, Kodak Adobe RIP, Agfa, Dainippon Screen, Creo Scitex Prinergy, Scitex Brisque, Rampage, TIFF bitmap files, and others.
The halftone screen may beat against the number of writing channels in the proofer creating a low frequency banding that is objectionable in the output proof. For example with a Kodak Approval XP4, direct digital color halftone proofer, writing a 200 line screen halftone at 45 degrees with a 2540 dot per inch printer, each halftone dot is nine micro-dots wide. If all 28 writing channels are used then 1/9th of a dot is left over every swath creating a beat frequency that is nine swaths wide. The swath period is 28 dots at 10 μm per dot for a total width of 280 μm. The beat frequency is (1 cycle)/(9×0.280 mm)=0.397 cycles per mm. If instead we image with 27 writing channels then each swath contains exactly three halftone dots and there is no beating against the writing width of the printer. The technique of setting the number of writing channels to eliminate the banding between the halftone screen and the printer is described in U.S. Pat. No. 5,329,297.
The problem with selecting the number of writing channels to eliminate beating between the printer and the halftone screen is to determine the optimum number of channels to use for each screen ruling and angle combination. Currently each halftone screen is imaged using each combination of numbers of channels, and the number of channels is selected that create the least amount of banding in the image. The process takes numerous proofs to complete. One proof is made for each selection of number of channels for each color plus an additional proof to study the final selection. The process requires a skilled operator to reconfigure the N-channel tables within the printer prior to imaging each proof. The process requires a skilled operator to evaluate the images and select the best number of channels.
The process of selecting the number of channels is repeated for each raster image processor (RIP), each dot shape, each screen ruling, each screen angle, and each color combination. If the RIP vendor modifies his screening algorithms then the optimization process needs to be checked to guarantee that the printer is setup at the optimum conditions. If a customer uses a RIP with a screen ruling and angle combination that has not been tested then the printer may not be optimized for the customer conditions.
In addition, the interaction between the writing channels and the halftone screen are dependent upon the magnitude of the errors within the group of writing channels. Higher quality printers with no errors produce less visible banding artifacts. These errors may include dot placement, printhead movement, channel power adjustment, and channel to channel crosstalk.
Thus, there exists a need to improve the process of selecting the number of channels used to image a given halftone screen ruling, screen angle, and RIP combination.